Arrangement and method for determining the oxygen buffer capacity in a catalytic converter

ABSTRACT

The invention relates to an arrangement for determining the oxygen buffer capacity in a catalytic converter (1) of an exhaust system. The arrangement includes an oxygen sensor (3, 32, 33) located downstream of at least one converter matrix (30, 31), where the sensor (3, 32, 33) supplies an input signal to a fuel injection control unit (4). The arrangement includes an analyzer circuit (13) for determining time information (TP) about the fluctuation of said input signal (25, 26), and calculating means for calculating the oxygen buffer capacity based on said time information (TP) when the downstream sensor is used to provide a primary control input signal (26). The invention also relates to a method of determining the buffer capacity.

FIELD OF THE INVENTION

The invention relates to an arrangement of the type defined in thepreamble of claim 1 and to a method as defined in the preamble of claim7 and 8 for determining the oxygen buffer capacity of a catalyticconverter.

The invention also relates to a method for controlling the air/fuelratio of a vehicle engine, based on the determination of the oxygenbuffer capacity.

BACKGROUND TO THE INVENTION

Catalytic converters for vehicle exhaust systems are well known. Suchconverters are typically constructed by having a housing within whichone or more converter bricks (often called converter matrices) arearranged. Each of the bricks has a washcoat containing one or more raremetals typically chosen amongst the elements of platinum, rhodium orpalladium.

The washcoat provides a plurality of catalytic reaction sites on whichoxygen which is temporarily stored within the catalytic converter as abuffer (i.e. an amount of surplus oxygen for later use) during leanmixture running, can undergo catalytic oxidation reactions with one orall of the following gases: carbon monoxide (CO), hydrocarbons (HC) andnitrous oxides (NO_(x)) of various types. Where a three-way catalyticconverter is provided, all the gases undergo oxidation reactions.

It has been observed previously however that the oxygen buffer capacityof the catalytic converter diminishes with time. In turn, the conversioncapabilities of the catalytic converter become steadily reduced suchthat the catalytic converter will be unable to cope with largefluctuations in the value of A (commonly adopted measure of air/fuelratio). Such large fluctuations may occur for instance when the engine'sfuel injection control unit alters the injection time so as to achievestoichiometric combustion conditions in the engine during accelerationor deceleration.

It is known that by taking a reading of the outputs of the front andrear oxygen sensors (typically so-called "lambda" sensors) andprocessing these values, a depleted buffer capacity can be detected thusallowing a deteriorated catalytic converter to be replaced in good time.

One such method for determining the presence of a reduced buffercapacity of a catalytic converter is disclosed in e.g. DE-A-38 30 515,in which the difference in the oxygen content of the exhaust gas bothupstream and downstream of the catalytic converter is measured. Bycomparing the quotient of this difference to the oxygen quantityupstream of the catalytic converter, a value is obtained which can becompared to known values. On the basis of the comparison, the conditionof the catalytic converter is determined so that replacement of thecatalytic converter at an appropriate time can be instigated.

In a further prior art device for determining catalytic deterioration asdisclosed in U.S. Pat. No. 5,228,335, the oxygen content signals from anoxygen sensor upstream and an oxygen sensor downstream of the catalyticconverter are fed into a microprocessor and compared to thresholdvalues. On the basis of said comparison, the deterioration level isdetermined.

The aforementioned prior art methods thus rely on the detection of aspecific set of conditions arising in order to be able to determinecatalytic converter failure.

The present invention aims at providing an arrangement and method fordetermining the size of the oxygen buffer capacity of a catalyticconverter, in particular at repeated intervals during engine operationso that updated information on buffer capacity is available.

In a further aspect of the invention, the value of the oxygen buffercapacity is used to provide a control input to the fuel injectioncontrol unit to alter the fuel injection to said engine in order tocompensate for reduced buffer capacity. Any method of measuring oxygenbuffer capacity can be used to provide the required information for suchcompensation.

SUMMARY OF THE INVENTION

The features of the present invention are defined in the independentclaims. Preferred features of the invention are defined in the dependentclaims. Further advantageous features also appear in the followingdescription.

In accordance with the invention, use is made of the signal which isemitted by the downstream oxygen sensor of a catalytic converter system.

As is known per se, the signal output of the upstream sensor of acatalytic converter system is one which fluctuates very rapidly andbasically periodically between high and low voltage output. This outputis used as a primary control input for a fuel injection control systemto control whether an increase or decrease in the amount of fuel shouldbe supplied to a fuel injector (i.e. whether a richer or weaker mixtureis required) Due to the fact that the upstream oxygen sensor tries toprovide almost stoichiometric combustion conditions, the output of thedownstream sensor is a fairly even, or slowly varying voltage, as itprovides only a fine-tuning of the air/fuel ratio required.

However, in accordance with the invention, the downstream sensor is usedto provide the primary input signal to the fuel injection control unitinstead of the upstream sensor. In this way, the adjustment of theoxygen content of the exhaust gases coming through the catalyticconverter will cause a rapidly fluctuating output signal from thedownstream sensor.

By analyzing aspects of the time function of this output signal (whichis also an input signal for a fuel injection control unit) the actualsize of the oxygen buffer capacity can be determined. The method ofanalysis will be explained in more detail below.

Since the oxygen buffer capacity is reduced during the life of acatalytic converter, the time taken to use up the buffer, and then torefill it with more oxygen from the exhaust gases during normal enginerunning, will be correspondingly reduced. Additionally, when the primaryinput to the fuel injection control unit comes from the downstreamsensor, the periodic time taken for the sensor to send a signal torichen or to weaken the air/fuel mixture will be relatively longcompared to that during normal operation of the upstream sensor. This isdue to the fact that the exhaust gases take a finite time to passthrough the converter and also a finite time to use up the stored oxygenbuffer. Consequently it will be apparent that when the oxygen buffercapacity is reduced, the periodic time of the fluctuations in the signalfrom the downstream sensor will be similarly reduced. In one embodimentof the invention, this reduction in periodic time is measured andcompared to known values of periodic time for different sizes of oxygenbuffer capacity. In this way the buffer capacity can be closelydetermined.

As an alternative method of using time information of the signalsemanating from the oxygen sensor, the output signals of both theupstream sensor and the downstream sensor may be used to determine theoxygen buffer capacity by measuring their time displacement(phase-shift). This is carried out in a similar manner to the above.Namely, the downstream sensor is again used to supply the primary inputto the fuel injection control unit and then the outputs of the upstreamand the downstream sensor are analyzed. The outputs of the upstream anddownstream sensors will be very similar, although as will be seen in theaccompanying figures, the signal of one is delayed in time with respectto the other. Measuring the time displacement will thus reveal theoxygen buffer capacity when compared to known values of timedisplacement.

The time during which the downstream sensor is used to supply theprimary control input may be as short as one time period, but willtypically be four or more time periods so that a mean value of timeperiod can be calculated and so that any transient effects fromnon-oxidised gases will be reduced.

In order to allow the downstream sensor to provide the primary controlinput for the fuel injection control unit, a switching means of somekind is provided. This switching means will typically be an electronicswitching means which is responsive to a series of control parameters.

Additionally, in order to be able to compare measured time information(e.g. periodic time of the fluctuations) with known values, themeasurements should occur within a so-called window of parameters. Forexample, when considering the parameters of engine load and speed, aparticular engine load and engine speed may result in a different set ofmeasured values from a different engine load. Similarly if the engine isnot operating at a steady temperature (typically during the warm-upphase) the readings will be different from when the engine is operatingat a steady temperature. Thus at least one window of pre-determinedparameters should be defined, in which the signal sampling is conducted.Typically the window will be within the normal driving cycle parameterrange of the engine. A plurality of windows may also be provided so thatit is easier to have sampling during each vehicle use.

Since however it is not possible to provide an infinite number ofwindows, a correction factor should be applied where possible. Inparticular, it has now been found that the mass flow (of fuel and air,or air only) affects the time period of the signal fluctuations in anapproximately linear manner. Thus an adjustment can be made such thatthe signal sampling can be carried out under virtually any known massflow conditions with good correspondence. In some cases the oxygenstorage is dependent on the temperature of the catalytic converter andan adjustment may also be required for this reason.

The measure of oxygen buffer capacity can be used to indicate thepresent condition of the catalytic converter and/or when replacement isrequired. Moreover, the results may however be used to control the fuelinput to the engine so that the gas content of the exhaust gases stayswithin the acceptable limits for the buffer capacity which has beendetermined. This fuel injection input control is thus a compensationinput control which will lead to reduced emissions in vehicles havingcatalytic converters with reduced oxygen buffer capacity.

This is due to the fact that by not using up all the oxygen buffer, i.e.by lying within the limits of the oxygen buffer somewhere close to themiddle thereof when catalytic reactions take place, the catalyticconverter is better able to take care of transients. Thus themodification of the air/fuel ratio from λ=1 to a lower value (i.e. to aricher mixture) for a limited period of time may be required to reducethe oxygen content of the available oxygen buffer capacity.

It should be observed that the expression "fuel injection control unit"may relate to a single control unit system, but that several controlunits may be included within said control unit so as to perform therequired control functions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference tocertain preferred embodiments and with reference to the accompanyingdrawings, in which:

FIG. 1 is a block diagram showing a typical arrangement for use inaccordance with the invention;

FIG. 2 is a block diagram of one of the elements in FIG. 1;

FIG. 3 is a typical flow chart for sampling a signal output from anoxygen sensor;

FIG. 4 depicts a typical set of signal outputs from an upstream and adownstream sensor before during and after switching the primary controlinput to be that output taken from the rear sensor;

FIG. 5 depicts a typical graph showing the reduction of time period withaging of a catalytic converter, and

FIG. 6 shows how the periodic time of the fluctuations varies with massflow (engine speed×load).

DESCRIPTION OF PREFERRED EMBODIMENTS

The arrangement shown in FIG. 1 comprises a catalytic converter havingtwo converter matrices 30, 31. The invention may however be carried outwith any number of matrices. The converter 1 has an inlet on the lefthand side (as depicted) and an outlet on the right hand side. Althoughthe connection is not shown, the inlet to the catalytic converter 1 isconnected to an exhaust outlet of an engine 9.

An oxygen sensor 2 is placed at the upstream end of the catalyticconverter and a further oxygen sensor 3 is placed downstream of thecatalytic converter in the direction of flow of exhaust gases. Thedownstream sensor may however be placed at any other location downstreamof the first sensor as long as it is also downstream of at least onecatalytic converter matrix. Thus alternative positions 32 and 33 for thedownstream sensor are shown in dashed lines.

For the sensor location 33, it is presumed that there is a convertermatrix between the sensor 2 and the matrix 30 as is known per se incertain systems.

Each of the sensors has an output 2a, 3b which is transmitted via aconnection 6 and 7 respectively to a fuel injection control unit 4. Theconnections 6 and 7 are typically electrical conductors, but radiosignals or light-transmitting cables could be used. Said outputs 2a and3b then provide respective input signals at inputs 4a and 4b of thecontrol unit 4. An output 4c of unit 4 is then connected via line 8 toan input 5a of a fuel injector means 5 (e.g. a fuel injector and/or afuel pump) which, in turn, supplies fuel to engine 9 via line 10. Line10 would normally be part of the inlet manifold unless direct injectionis involved.

During normal operation (i.e. all operation apart from when a testsampling of the signal is to be performed), the upstream sensor 2 isused to provide a primary control voltage input to the unit 4. On thebasis of this input, a voltage is fed through line 8 so as to controlwhen, and for how long, fuel is to be supplied by injector 5 to theengine 9. The control is normally set up so that, on the basis of theoxygen quantity in the exhaust gas present at sensor 2, a ratio of closeto λ=1 should be achieved. Similarly, the downstream sensor 3, duringnormal operation, finely adjusts the air/fuel ratio based on the oxygencontent of the exhaust gases after passage through the catalyticconverter.

FIG. 2 shows three basic elements contained within the control unit 4; awindow parameter circuit 11, a selector circuit 12 and an analysiscircuit 13. The circuit 11 decides when the system should be operatingnormally and when the system should be run to determine the oxygenbuffer capacity. In order to decide this, the circuit 11 is set up so asto test whether a number of window parameters are fulfilled beforeproceeding with a test of the oxygen buffer capacity. Such parametersmay for example include engine temperature, oil temperature, catalyticconverter temperature, engine running time which has elapsed afterinitial engine ignition, engine running time since the last buffercapacity test and/or number of tests carried out in any onestart-to-stop operation. Whilst further factors such as mass flow offuel to the inlet side of an engine, mass flow of air to the inlet sideof an engine, engine rotational speed and engine load may be included aswindow parameters, a linear correction can be made for any known valueof these and thus such are not normally included in the window. However,constant engine speed and constant engine load may typically be windowrequirements to avoid transient effects.

A switching means is provided in the form of a selector circuit 12. Whenthe window parameters are fulfilled, a signal is sent to the circuit 12and a switching occurs such that the signal from the downstream sensor 3is used as the primary input to the unit 4. This is thus equivalent tomaking the input 4b take the place of input 4a. In this way, the outputsignal from the downstream sensor will fluctuate in order to provideprimary control of the fuel/air mixture.

The signal is analyzed by circuit 13 and the time information value(s)from the signal is/are compared to one or more predetermined value(s) soas to arrive at a measure of the oxygen buffer capacity. The circuit 13may be additional to, and/or part of, the normal circuitry for providinga fuel injection control output via line 8 (output 4c). The analysisperformed by circuit 13 is normally a measurement of the periodic time(time period), or average periodic time, of the fluctuations in theoutput signal from the downstream sensor. When the downstream sensor isused as the primary control input, the time period of the upstreamsensor however also changes and thus either one may in fact be measured.As an alternative, the time information to be used can be the timedisplacement between the input signals from the upstream and thedownstream sensor.

A calculation of the oxygen buffer capacity can then be made by acomparison calculation with known values of period time for variousbuffer capacities. Thus the actual buffer capacity can be determined asa particular time length, or the percentage buffer capacity remainingcan be determined by comparison with corresponding values.

FIG. 3 shows a typical flow chart for a determination of the buffercapacity. A series of input parameters such as engine speed, load,temperature and/or time, are sent to window parameter block 11. In block16 it is determined whether the window parameters of any window arefulfilled or not. If they are not fulfilled, block 17 is followed sothat the air/fuel ratio continues to be adjusted on the basis of aprimary signal from the upstream sensor. This will be the normalsituation.

When the parameters of a window are fulfilled, block 18 indicates that asignal to initiate a test will be sent to selector block 12 in controlunit 4. This will initiate block 19 which makes the control unit 4receive the primary control input from input 4b such that fuel injectioncontrol is effected by the downstream sensor 3. The fluctuations of thesignal input at 4b or 4a will then be analyzed (cf. block 20) in orderto measure the time period of the fluctuations. Alternatively the signalinputs at inputs 4a and 4b will be analyzed to measure the timedisplacement between the two signals. Block 21 signifies that adetermination of oxygen buffer capacity will then be made, this being afunction of the analyzed time information, and hence the symbolism K(t).In block 22 a decision is made as to whether the analysis is complete.In the affirmative (YES), the analysis is terminated and the controlunit 4 will assume normal operation. If negative (NO), the steps carriedout in blocks 20 to 22 will be initiated again.

The flow chart is however only explanatory and it will be clear thatmany further possible alternative flow charts will be evident to the manskilled in the art.

The upper portion of FIG. 4 shows typical voltage output signals 23 and25, and 24 and 26, from the front and rear oxygen sensors respectively,with respect to time (t). The lower portion of FIG. 4 shows an outputvalue 27 corresponding to the typical applied factor for fuel injectiontime and thus how the length of injection timing can vary.

The graph is divided into three time zones 20, 21 and 22. Zone 20 is azone in which the engine is running as normal with the primary inputfrom the upstream sensor. Zone 21 is the analysis time zone with primarycontrol input from the downstream sensor 3 and zone 22 is when theprimary control input is switched back to the upstream sensor 2.

The output in zone 21 corresponds to a typical output measured overapproximately ten seconds, although shorter or longer time zones can beused for zone 21.

In zone 20, the upstream sensor output 23 fluctuates rapidly whichmodifies the air/fuel ratio to lean and rich and then back again. Thedownstream sensor 3 output does not fluctuate rapidly but insteadsupplies a gently varying output to the fuel control unit 4 to richen orweaken the mixture by small increments.

In zone 21, the time period and size of the signal of the downstreamsensor output 26 vary relatively rapidly as it alters the inlet mixturefrom weak to rich and back again. The output of the upstream sensor alsofollows this with a very similar fluctuation waveform, although with adistinct and relatively constant time lag compared to signal 25. As theoxygen buffer capacity decreases with an aging catalytic converter, theperiod of each of the signals becomes shorter and the time lag becomesless. Thus either of these time information sources may be used for thesignal analysis. In zone 22 the output of the two sensors returns to itsnormal value as in zone 20. A zone similar to zone 21 will then recurwhen the window parameters are again fulfilled.

The fluctuating signal 27 shows a value corresponding to the typicalapplied factor for the injection time. This signal also varies in zone21 since it is dependent on the resultant input signal from the rearsensor. In this way, the periodic time of the signal 27 in zone 21 canalso be used to determine the buffer capacity. The signal 27 may haveother fluctuation shapes (e.g. square wave form, etc.).

FIG. 5 shows how the periodic time TP varies with the number of milesdriven for a typical catalytic converter. Thus with a new catalyticconverter, TP is almost five seconds whereas with an aged catalyticconverter with 200,000 driven miles the time period has been reduced toabout half. As is clear, TP reduces with the number of driven miles onan almost linear scale in this particular exhaust system. Since thisreduction is due to the reduction in oxygen buffer capacity, a directcomparison can be made with known buffer capacity data in order todetermine the present buffer capacity. Different curves will be producedfor different vehicles.

FIG. 6 shows how the periodic time TP varies linearly with mass flow ofair or fuel/air into the engine. In the case shown, the vertical axis ismarked N*TL,

where N=engine speed and TL=load, the product of these two factorsexpressing mass flow. By using this graph, time period analysis can becarried out in accordance with the aforementioned method and arrangementat any known engine speed and load with a corrected value of periodictime being obtained for comparison with known values for certain buffercapacities. As explained above, this generally allows the factors ofengine speed and load to be left out of the window parameters. However,engine tickover speed may cause some problems and thus a windowparameter may be included so as to exclude any unwanted analysis below1200 r.p.m. for example.

Once the oxygen buffer capacity is known, this value can be stored in amemory for subsequent use. By using this stored data, the fuel injectionto the engine can then be modified so that the amount of oxygen buffercapacity never becomes depleted or completely full. This is achieved,for example, by using a short period of weakening and/or richening ofthe air/fuel mixture at appropriate intervals.

Richening may be achieved by a small extra quantity of fuel beinginjected so as to richen the mixture temporarily which will result inmore hydrocarbons being passed into the exhaust gases so as to preventthe oxygen in the exhaust gases from filling the entire oxygen buffercapacity. The fuel may be injected by, for example, a marginally longerinjection period compared to that required.

Similarly, mixture weakening can be performed in order to help fill theoxygen buffer capacity if it is being reduced.

The size of such buffer corrections will be made on the basis of thedetermined buffer capacity. Typically these corrections will occurduring acceleration or deceleration phases and/or shortly thereafter.

What is claimed is:
 1. A system for determining the oxygen buffercapacity of a catalytic converter comprising:said catalytic converterincluding a converter matrix; a downstream oxygen sensor locateddownstream from said converter matrix, said downstream oxygen sensorgenerating a first signal; an upstream oxygen sensor located upstreamfrom said converter matrix, said upstream oxygen sensor generating asecond signal; a fuel control unit in communication with said catalyticconverter for receiving said first and second signals, wherein saidsecond signal normally provides a primary input signal for controllingsaid fuel control unit, said fuel control unit providing an outputsignal for determining an air/fuel ratio of said system, said outputsignal being derived from said first and second signals, said fuelcontrol unit including a switching element for selectively switchingfrom a first state in which said second signal provides said primaryinput signal for said fuel control unit to a second state in which saidfirst signal provides said primary input control signal for said fuelcontrol unit and an analyzer circuit for determining time informationabout said signal; a calculating element for calculating the oxygenbuffer capacity of said catalytic converter in response to said timeinformation; and wherein said fuel control unit alters the air/fuelratio of the system in response to the oxygen buffer capacity determinedby said calculating element.
 2. A system according to claim 1, whereinsaid analyzer circuit compares the frequencies of said first and secondsignals so as to determine differences between said first and secondsignals.
 3. The system as claimed in claim 1, said first and secondsignals having respective frequencies, wherein said analyzer circuitcompares the respective frequencies of said first and second signals soas to determine phase differences between said respective frequencies ofsaid first and second signals.
 4. The system as claimed in claim 1,wherein said analyzer circuit measures the periodic time (TP) of thefrequencies in said first and second signals and a frequency signalderived from said first and second signals.
 5. The system as claimed inany one of claims 2, 3 or 4, said fuel control unit further including awindow parameter circuit for identifying one or more predeterminedparameters of said system, said window parameter circuit generating anoutput signal when one of said predetermined parameters is identifiedfor activating said switching element.
 6. The system as claimed in claim1, said system further comprising a memory for storing the oxygen buffercapacity, wherein an air/fuel mixture ratio of said system isestablished in response to said oxygen buffer capacity.
 7. A method fordetermining the oxygen buffer capacity of a catalytic converter in asystem comprising:providing a catalytic converter having a convertermatrix; providing an oxygen sensor downstream from said convertermatrix; providing an oxygen sensor upstream from said converter matrix;generating a first signal from said downstream oxygen sensor and asecond signal from said upstream oxygen sensor; providing a fuel controlunit in communication with said catalytic converter and supplying saidfirst and second signals to said fuel control unit; using said first andsecond signals to control the air/fuel ratio of the system; switchingfrom a first state in which said second signal provides a primary inputsignal for said fuel control unit to a second state in which said firstsignal provides said primary input control signal for said fuel controlunit; analyzing said first and second signals to determine timeinformation about said first and second signals and using said firstsignal as said primary input signal for said fuel control unit whenanalyzing said first and second signals; calculating the oxygen buffercapacity of said catalytic converter as a function of said timeinformation when said first signal provides said primary input controlsignal for said fuel control unit; and altering the air/fuel ratio ofsaid system in response to the calculation of the oxygen buffercapacity.
 8. The method as claimed in claim 7, wherein said timeinformation includes the frequency of said first signal.
 9. The methodas claimed in claim 7, wherein the time information includes therespective frequencies of said first and second signals.
 10. The methodas claimed in claim 9, wherein the time information includes phasedifferences between said respective frequencies of said first and secondsignals.
 11. The method as claimed in claim 7, said fuel control unithaving a window parameter circuit for identifying one or morepredetermined parameters of said system, said method comprising thesteps of:evaluating said first and second signals so as to identify whenone of said predetermined parameters has been achieved; and generatingan output signal from said window parameter circuit when one of saidpredetermined parameters is achieved.
 12. The method as claimed in claim11, wherein said predetermined parameters are selected from the groupconsisting of the parameters of mass flow of fuel to an inlet side of anengine, mass flow of air to the inlet side of said engine, enginerotational speed, engine load, engine temperature, oil temperature,catalytic converter temperature, engine running time elapsed afterinitial engine ignition and engine running time since the last buffercapacity test.
 13. The method as claimed in claim 7, the altering stepincluding the step of enriching said air/fuel ratio.
 14. The method asclaimed in claim 13, wherein the enriching step includes injecting fuelinto said system over a lengthier period of time than normally requiredto reach stoichiometric conditions.
 15. The method as claimed in claim13, wherein the enriching step includes supplying an additional quantityof fuel.
 16. The method as claimed in claim 7, further comprising thestep of weakening said air/fuel ratio by supplying said fuel over ashorter fuel injection time than normally required to reachstoichiometric conditions.
 17. The method as claimed in claim 16,further comprising the step of weakening said air/fuel ratio bysupplying a lower quantity of fuel.